KATHMANDU -- Dozens of endangered Great One-horned rhinoceros have mysteriously gone missing from a nature reserve in southwest Nepal over the past few years, a wildlife official said on Wednesday.

Authorities introduced 72 rhinos, also known as the Indian rhinoceros, in the Babai Valley, 320 km (200 miles) southwest of Kathmandu, as part of a conservation drive that started in 1984.

"We have records showing 23 rhinos had died due to poaching or other causes. The rest are missing," Laxmi Prasad Manandhar, a senior official at the Department of National Park and Wildlife Conservation, said.

But he ruled out the possibility of all the 49 missing rhinos falling prey to poachers.

"If poachers had killed them they should have left behind the bodies" after taking away the horn, he said, adding that just one rhino skeleton had been found during an extensive search in June.

"Where did they go? I have no answer. It is a mystery," Manandhar said.

The rhinos were moved to Babai Valley from Chitwan National Park on Nepal's southern plains under a conservation scheme supported by global conservation group WWF.

In December, Nepal's Supreme Court ordered the government to step up security at Chitwan -- the Himalayan nation's biggest rhino reserve -- after local media reported at least 10 animals had been killed since July.

Officials say at least 12 rhinos had died in the past six months in Chitwan where their population dropped to 372 in 2005 from 544 in 2000.

Their numbers fell mainly due to poaching for horns which are believed to have aphrodisiac qualities and are in great demand in China.

In the Babai Valley, rhinos were last seen seven years ago when several security posts were closed due to threats from the Maoist rebels who targeted them during their decade-long insurgency against Nepal's monarchy.

The Maoists declared a cease-fire in April and signed a peace deal with the government in November, allowing easier and safer movement of forestry officials.

Nepal began its rhino conservation drive 30 years ago when the population fell to 108 animals from around 800 in 1950. One-horned rhinos are also found in the northeastern Indian state of Assam.

The one-horned species of the rhinoceros has been one of the greatest conservation success stories in South Asia. With strict protection, especially in India, their total numbers have touched around 2,500 from 100 about a century ago.